From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,UNPARSEABLE_RELAY autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DD02DC43142 for ; Sat, 23 Jun 2018 01:01:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 93E9024A66 for ; Sat, 23 Jun 2018 01:01:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 93E9024A66 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934470AbeFWBBg (ORCPT ); Fri, 22 Jun 2018 21:01:36 -0400 Received: from out30-132.freemail.mail.aliyun.com ([115.124.30.132]:52943 "EHLO out30-132.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933773AbeFWBBf (ORCPT ); Fri, 22 Jun 2018 21:01:35 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R581e4;CH=green;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01f04455;MF=yang.shi@linux.alibaba.com;NM=1;PH=DS;RN=13;SR=0;TI=SMTPD_---0T3CFQK-_1529715669; Received: from US-143344MP.local(mailfrom:yang.shi@linux.alibaba.com fp:SMTPD_---0T3CFQK-_1529715669) by smtp.aliyun-inc.com(127.0.0.1); Sat, 23 Jun 2018 09:01:15 +0800 Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping From: Yang Shi To: Michal Hocko , Nadav Amit Cc: Matthew Wilcox , ldufour@linux.vnet.ibm.com, Andrew Morton , Peter Zijlstra , Ingo Molnar , acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, "open list:MEMORY MANAGEMENT" , linux-kernel@vger.kernel.org References: <1529364856-49589-1-git-send-email-yang.shi@linux.alibaba.com> <1529364856-49589-3-git-send-email-yang.shi@linux.alibaba.com> <3DDF2672-FCC4-4387-9624-92F33C309CAE@gmail.com> <158a4e4c-d290-77c4-a595-71332ede392b@linux.alibaba.com> <20180620071817.GJ13685@dhcp22.suse.cz> Message-ID: <94bdfcf0-68ea-404c-a60f-362f677884b6@linux.alibaba.com> Date: Fri, 22 Jun 2018 18:01:08 -0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Yes, this is true but I guess what Yang Shi meant was that an userspace >> access racing with munmap is not well defined. You never know whether >> you get your data, #PTF or SEGV because it depends on timing. The user >> visible change might be that you lose content and get zero page instead >> if you hit the race window while we are unmapping which was not possible >> before. But whouldn't such an access pattern be buggy anyway? You need >> some form of external synchronization AFAICS. >> >> But maybe some userspace depends on "getting right data or get SEGV" >> semantic. If we have to preserve that then we can come up with a VM_DEAD >> flag set before we tear it down and force the SEGV on the #PF path. >> Something similar we already do for MMF_UNSTABLE. > > Set VM_DEAD with read mmap_sem held? It should be ok since this is the > only place to set this flag for this unique special case. BTW, it looks the vm flags have used up in 32 bit. If we really need VM_DEAD, it should be for both 32-bit and 64-bit. Any suggestion? Thanks, Yang > > Yang > >